He has a lot of attached adds that can be mowed down pretty easily. The issue with G is the amount of damage caused by his spellcast dumbfire pets.
Notes about the targeted summoned pets:
- Killing them heals Gnorbl.
- Attempts at curing the arcane effect (the pets) were unsuccessful
- They can be mezzed
- The pets taunt, causing you lose your current target and to target them.
- The pets disappear if their initial the target of the cast dies
- They hit for massive crushing damage
- Seems to cast them every 30 seconds or so
It's important for those that are in taunt range of the dumbfire pets to create a /target "Gnorble the Playful" button.
Mow down the tots and try to keep the dumbfire pets mezzed as much as possible, spamming your target key so you don't get taunted off of Gnorbl.